This is a 317 of PCT/F198/00667 filed Aug. 26, 1998.
The invention relates to a method of preparing crystalline L-arabinose by extraction of sugar beet pulp, from which sugar has been extracted, in a strong alkaline solution, by hydrolysis of the obtained crude araban with a strong acid at an elevated temperature, by chromatographic separation of the L-arabinose fraction, by purification of the obtained L-arabinose solution by means of cation and anion exchangers and adsorbent resins, and by recovering the pure L-arabinose as a crystalline product.
L-arabinose has been prepared by acid hydrolysis from arabinose-containing vegetable materials, such as gum arabic. Another well known raw material is sugar beet pulp from which sugar has been extracted; L-arabinose has been prepared from this material by alkaline or acid hydrolysis followed by multistep purification. U.S. Pat. No. 4,816,078, for example, teaches a method of preparing L-arabinose from sugar beet pulp by hydrolysis in the presence of lime, by filtration and chromatographic separation of the araban fraction; whereupon the araban is subjected to hydrolysis by addition of acid, and the obtained L-arabinose is separated by chromatography using a cation exchanger in Ca-form. According to a previous method described in GB published application 1,182,099, L-arabinose is prepared from a beet pulp hydrolyzate by crystallization from an alcoholic solution.
It has now been discovered that L-arabinose can be obtained with good yield from sugar beet pulp, from which sugar has been extracted, by simple alkaline extraction, by acid hydrolysis and chromatographic separation using as separating resin a cation exchanger in monovalent metal (Na+) form, to obtain an L-arabinose-containing fraction. As a last step, the L-arabinose is crystallized from an aqueous solution. The method avoids the previous multiple separation and purification steps.
The method of the invention enables the preparation of pure L-arabinose. In accordance with the invention, sugar beet pulp, from which sugar has been extracted, is extracted with an alkaline solution to dissolve the araban. After filtration the alkaline extract is hydrolyzed with acid to obtain a crude L-arabinose solution. The crude arabinose fraction is subjected to chromatographic separation by using a cation exchanger resin in monovalent metal (Na+) form (e.g. sulphonated polystyrenedivinyl benzene; 5.5% DVB) and water as eluent. The solution is then purified by ion exchange. The solution is concentrated to about 70 percent by weight, and the pure L-arabinose is crystallized. The crystals are separated by centrifugation and dried in air.
The separating resin used in the chromatographic separation is preferably sulphonated polystyrenedivinyl benzene (5.5% DVB) in monovalent metal (Na+, K+) form. Purification and colour removal is carried out with cation and anion exchange resins and other adsorption resins.
As feedstock is preferably used sugar beet pulp stabilized by the biotechnical method according to Finnish Patent Application 973,501. The following presents a general flow chart.
In accordance with a preferred embodiment of the invention, sugar beet pulp, from which sugar has been extracted, is suspended in a strong alkaline solution and extracted for 4 to 5 hours at a pH of about 10 to 12 and a temperature of 95 to 100xc2x0 C. The alkaline xe2x80x9csolutionxe2x80x9d is preferably milk of lime containing 25 to 40 percent by weight of Ca(OH)2 calculated on the dry substance of the sugar beet pulp. Optionally, buffered alkaline solutions (KOH, NaOH) can be used in the extraction. The obtained mixture is neutralized with e.g. carbon dioxide or optionally sulphuric acid to about pH 9 or below to precipitate the calcium salt. After filtration the solution is concentrated to about 40 percent by weight and then subjected to acid hydrolysis, e.g. by addition of sulphuric acid, to pH about 0.8. The hydrolysis is carried out at about 90xc2x0 C., whereupon the solution is cooled to about 70xc2x0 C. and neutralized by addition of an alkali (e.g. NaOH) to pH about 6. The neutralized solution is subjected to chromatographic separation by using a cation exchange resin in monovalent metal form, preferably Na+ form. The separated arabinose-containing fraction has a dry substance content of 5 to 10%, of which 75 to 80% is L-arabinose. Pure L-arabinose crystals are obtained by evaporation (a solution of about 70 percent by weight), seeding and cooling crystallization. The crystals can be separated by centrifugation.
Another preferable way to extract and hydrolyze araban is to continue by acid hydrolysis after alkaline hydrolysis to degrade the araban into L-arabinose. In this embodiment the alkaline extraction takes about 4 to 5 hours, whereupon the pH is adjusted to about 0.8 with sulphuric acid. The extract is treated at 90xc2x0 C. until the araban is degraded, whereupon the suspension is neutralized (pH 6), filtered by a filter press and concentrated by evaporation for chromatographic separation.
The choice of embodiment depends on equipment capacity and the suitability of the raw material.